A power amplification circuit in a wireless system is typically a large signal device. In wireless local area network (WLAN) systems, the power amplifier circuit may transmit output signals at average power levels in the range of 10 dBm to 21 dBm, and peak power levels of about 25 dBm, for example. In WLAN systems, which use orthogonal frequency-division multiplexing (OFDM) or complementary code keying (CCK) modulation, output power levels may vary widely such that the ratio of the peak power level to the average power level may be large, for example, 12 dB for OFDM and 6 dB for CCK. Because of these large swings in output power levels, power amplifier (PA) circuits may distort the output signal. Distortion, however, is a characteristic, which may be observed in PA circuits that are utilized across a wide range of applications, and may not be limited to PA circuits utilized in wireless systems. There are two metrics, which may be utilized to evaluate the distortion performance of PA circuits. These metrics may be referred to as amplitude modulation to amplitude modulation (AM-AM) distortion, and amplitude modulation to phase modulation (AM-PM) distortion.
The AM-AM distortion provides a measure of the output power level, pout, in response to the input power level, pin. In an ideal, non-distorting, PA circuit, the output power level changes linearly in response to a change in the input power level. The AM-AM distortion may be observed when, for example, the output power level does not change linearly in response to change in the input power level.
The AM-PM distortion provides a measure of the phase of the output signal in relation to the phase of the input signal (or phase change) in response to the input power level. The AM-PM distortion may be observed when, for example, the input to output phase-change varies in response to a change in input power level.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.